Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that of butenes is likely to increase, due to a developing shortage of propylene.
Thus, a process that uses butenes or higher alkenes instead of propylene as feed and coproduces methyl ethyl ketone (MEK) or higher ketones, such as cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenols. For example, there is a growing market for MEK, which is useful as a lacquer, a solvent and for dewaxing of lubricating oils. In addition, cyclohexanone is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known that phenol and MEK can be produced from sec-butylbenzene, in a process where sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the peroxide decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described in pages 113-121 and 261-263 of Process Economics Report No. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
For example, in our International Patent Publication No. WO06/015826, we have described a process for producing phenol and methyl ethyl ketone, in which benzene is contacted with a C4 alkylating agent under alkylation conditions with a catalyst comprising zeolite beta or a molecular sieve having an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07 Angstrom to produce an alkylation effluent comprising sec-butylbenzene. The sec-butylbenzene is then oxidized to produce a hydroperoxide and the hydroperoxide is decomposed to produce phenol and methyl ethyl ketone. The oxidation step can be conducted with or without a catalyst under conditions including a temperature between about 70° C. and about 200° C., such as about 90° C. to about 130° C., and a pressure of about 0.5 to about 10 atmospheres (50 to 1000 kPa). Suitable catalysts are said to include the N-hydroxy substituted cyclic imides described in Published U.S. Patent Application No. 2003/0083527.
There is a need to find an oxidation process for sec-butylbenzene and cyclohexylbenzene that is highly selective to sec-butylbenzene or cyclohexylbenzene hydroperoxide, that is less sensitive to the presence of impurities than the existing oxidation processes, and that allows efficient commercial scale production of phenol and MEK or phenol and cyclohexanone.
It is known from, for example, U.S. Pat. Nos. 6,852,893 and 6,720,462 that certain cyclic imides, such as N-hydroxyphthalimide, in combination with free radical initiators, such as peroxy compounds or azo compounds, are effective catalysts in the catalytic oxidation of a wide variety of aliphatic or aromatic hydrocarbons, including alkyl aromatic hydrocarbons, such as cumene, cyclohexylbenzene, cyclododecylbenzene and sec-butylbenzene, to the corresponding hydroperoxides. The patents teach that the oxidation can be conducted over a wide range of process conditions including a temperature of 0 to 500° C. and a molar ratio of the catalyst to the hydrocarbon to be oxidized between 10−6 mol % and 10 mol %. However, no suggestion is provided in either patent as to the efficacy, or the preferred conditions, of the process for the selective oxidation of sec-butylbenzene to sec-butylbenzene hydroperoxide or cyclohexylbenzene to cyclohexylbenzene hydroperoxide.
U.S. Pat. No. 7,038,089 discloses a process for preparing hydroperoxides from their corresponding hydrocarbons which comprises oxidizing the hydrocarbons, particularly ethylbenzene, at a temperature in the range between 130 and 160° C. with an oxygen containing gas in the presence of a catalyst comprising a cyclic imide compound and an alkali metal compound. In particular, the '089 patent teaches that when oxidation of ethylbenzene is carried out in the presence of a catalytic comprising a cyclic imide and an alkaline metal compound, simultaneously high reaction rate and high selectivity to the corresponding hydroperoxide are obtained, superior to those which are obtained when both components from the catalytic system are used independently. In contrast, according to the '089 patent, when the cyclic imide alone is used as the catalyst, high imide concentrations have to be avoided for cost and product impurity reasons, but reducing the imide concentration to tolerable levels, requires a rise in temperature to increase reaction rate, leading to a decrease of the selectivity to hydroperoxide to unacceptable levels.
According to the present invention, it has now been found that, with sec-butylbenzene and cyclohexylbenzene, oxidation can be conducted at commercially viable conversion rates and sec-butylbenzene and cyclohexylbenzene hydroperoxide selectivities in the presence of a cyclic imide catalyst, without the addition of alkaline metal compound, provided the conversion is conducted over a relatively narrow range of temperature and cyclic imide concentration. Contrary to the teaching in U.S. Pat. No. 7,038,089, with sec-butylbenzene oxidation and cyclohexylbenzene, it has been found that the presence of an alkaline metal compound significantly reduces the activity and hydroperoxide selectivity of the oxidation catalyst.